As a display device for a cell-phone unit and a personal computer, a liquid crystal display device is used widely.
For example, the liquid crystal display device includes a pair of substrates arranged opposing each other, a liquid crystal layer held between the substrates, a display portion consisting of a plurality of display pixels arranged in the shape of a matrix, and a driving circuit to drive the display pixels.
Pixel electrodes are arranged on one of the substrates in the shape of a matrix so that the pixel electrode corresponds to each display pixel. On the other substrate, a counter electrode is arranged so as to oppose to the pixel electrodes. The alignment state of crystal molecules constituting the liquid crystal layer is controlled by a voltage applied between the pixel electrode and the counter electrode.
When a constant voltage (direct-current voltage) has been impressed to the liquid crystal layer for a long time, inclination of the liquid crystal molecules is fixed. Therefore, the liquid crystal molecular inclination of the liquid crystal layer causes persistence of image and further a short life. In order to prevent the unfavorable phenomenon, the voltage impressed to the liquid crystal layer is alternately changed for every definite period. That is, the voltage impressed to the pixel electrode is changed to a positive voltage side and a negative voltage side for every definite period with respect to the voltage applied to the counter electrode in the liquid crystal display device.
As a driving method for impressing the alternating current voltage to the liquid crystal layer, for example, Japanese Laid Open Patent Application No. 2005-173244 proposes a capacity coupling (CC) driving method. In the capacity coupling (CC) driving method, when the pixel switch is turned off (non-conductive state), amount of change of the pixel electrode potential is made larger than that of the signal voltage supplied to a signal line by controlling the voltage of an auxiliary capacity line.
In the liquid crystal display device using the capacity coupling driving method, when the driving circuit for driving the auxiliary capacity line is constituted by a CMOS circuit, the manufacturing process steps may increase. Conventionally, in order to prevent the increase of the manufacturing steps, Japanese Laid Open Patent Application No. 2006-276541 proposes a technique in which the driving circuit for driving the auxiliary capacity lines is formed of either one of PMOS transistor or NMOS transistor.
However, while the miniaturization of the transistor progresses by the development of the manufacturing process technology in recent years, when the transistor is OFF, an OFF leakage current may flow. When an excessive OFF leakage current flows into the transistor, the ON and OFF switching operation of the transistor in the auxiliary capacity line driving circuit could not be controlled normally, and the auxiliary capacity line driving circuit may malfunction.
Moreover, it is considered to make the value of a retention capacity connected with the auxiliary capacity line large to decrease the fluctuation of the potential of the lines in the floating state even if the excessive off-leakage current flows into the transistor. When the value of the retention capacity is enlarged, the width of the frame surrounding the display portion becomes large by the capacity area, and further the external dimensions of the liquid crystal display device module may also become large.
Furthermore, when writing a potential through the transistor into scanning lines, the writing of the potential may not complete, and the potential may not be written sufficiently during a high level period due to taking time. Also in this case, the auxiliary capacity line driving circuit may malfunction.